I don't know how the charge was worded or what definitions the government uses, but it is clearly more than just "malicious vehicular assault" (e.g. deliberately using a car as a weapon to hit someone).
The plan was to hit buildings and cause a fuel explosion. The plane was a bomb. Dead is dead.
Does the definition of "weapons of mass distruction" carry with it an intended body count range, a potential explosive damage yield, or a financial damage value?
Now that I have thought about my previous post, the charge that was thrown out was actually against shoe bomber Richard Reid. The charge related to attempting to destroy a vehicle of mass transit or how ever they worded it. The charge was thrown out because as congress wrote the law, airplanes didnt fall under the definition of a vehicle of mass transit.